Cut Soybean Plants and the Culprit Back »

Figure 1. Soybean plants cut by cutworm caterpillars.
Courtesy: John C. French, Bugwood.org


Written collaboratively by Adam Varenhorst, Patrick Wagner, and Amanda Bachmann.

We have received numerous reports of cutworms being problematic in safflower and sunflower so far in 2017. However, while scouting soybeans last week I noticed numerous areas within the field where soybean plants were lying dead near the row, but with no indication of defoliation. Upon closer inspection, it became evident that the plants had been cut near the soil surface. Reports of cutworms in soybean tend to be rare, but there are documented cases of them occasionally causing problems. The cutworm species that we found near the areas with cut plants was the dingy cutworm (Figure 1).

Dingy Cutworm: Identification

The dingy cutworm has been covered previously because of its presence in safflower and sunflower this year. The caterpillars are nocturnal, and can be found by digging 1-2 inches in the soil around cut plants. Dingy cutworms get their name due to their dull brown to gray coloration. These cutworms have a distinct pale gray line that runs down the center of their bodies. The dingy cutworm caterpillars have tubercles or spots present on the sides of each segment of their bodies that are all similar in size to one another (Figure 2).


Figure 2. Dingy cutworm caterpillar. Courtesy: J. Capinera, Bugwood.org
 

Scouting & Management

Cutworms tend to be more of an issue in soybean fields that have been planted under reduced or no-till practices, fair to poorly drained fields, or fields with weed or cover crop presence prior to soybean planting. Cutworms can be scouted by examining fields for cut plants and also by digging in the soil near cut plants to determine population densities. To scout for cut plants, examine 20 consecutive plants in five locations throughout the field (100 plants per field). In addition to looking for cut plants, also examine plants for signs of defoliation as younger cutworm caterpillars may be incapable of cutting the plant. In areas where cut plants are observed, dig in the soil within the row to find any caterpillars that may be present.

Insecticides
Insecticide management should be considered if 20% or more of the scouted plants are cut and cutworm caterpillars that are 3/4 of an inch or shorter are observed. Caterpillars this size will continue to feed on plants and may further reduce stands. Please refer to the current edition of South Dakota Pest Management: Soybean for a list of insecticides labeled for the management of cutworms in soybean.

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